Ishaan is 3 times as old as Christopher. Twenty years ago, Ishaan was 8 times as old as Christopher. How old is Christopher now?
Explanation: We can use the given information to write down two equations that describe the ages of Ishaan and Christopher. Let Ishaan's current age be $i$ and Christopher's current age be $c$ The information in the first sentence can be expressed in the following equation: $i = 3c$ Twenty years ago, Ishaan was $i - 20$ years old, and Christopher was $c - 20$ years old. The information in the second sentence can be expressed in the following equation: $i - 20 = 8(c - 20)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $c$ , it might be easiest to use our first equation for $i$ and substitute it into our second equation. Our first equation is: $i = 3c$ . Substituting this into our second equation, we get: $3c$ $-$ $20 = 8(c - 20)$ which combines the information about $c$ from both of our original equations. Simplifying the right side of this equation, we get: $3 c - 20 = 8 c - 160$ Solving for $c$ , we get: $5 c = 140.$ $c = 28$.